1. Field of the Invention
The present invention relates to an ultraviolet light monitoring system that quantitatively and in real time monitors damage caused by ultraviolet light from plasma or the like in a semiconductor fabrication step (process) or the like.
2. Description of the Related Art
With the progress of miniaturization of element structures, thinning of layers and formation of three-dimensional structures in, for example, semiconductor devices, ultraviolet light irradiation damage, when ultraviolet light is emitted from a plasma which is used in a step of forming an interlayer insulation film, wiring or the like and reaches a boundary surface of a semiconductor element, has become a significant problem.
As a countermeasure, technologies have been developed for ultraviolet light monitoring systems (for example, a plasma monitoring system) which monitor ultraviolet light irradiation damage in real time at actual patterns on wafers. Methods and evaluation results of these technologies are presented in detail in documents such as, for example:
Japanese Patent Application Laid-Open (JP-A) No. 2003-282546 (below referred to as document 1);
JP-A No. 2005-236199 (below referred to as document 2);
J. Vac. Sci. Technol. A, Vol. 23, No. 6, November/December 2005, American Vacuum Society, pp. 1509-1512 (below referred to as document 3);
J. Vac. Sci. Technol. B, Vol. 23, No. 1, January/February 2005, American Vacuum Society, pp. 173-177 (below referred to as document 4); and
J. Vac. Sci. Technol. B, Vol. 22, No. 6, November/December 2004, American Vacuum Society, pp. 2818-2822 (below referred to as document 5); and the like.
FIG. 8A and FIG. 8B are diagrams showing a conventional plasma monitoring system which is described in, for example, FIG. 6 of the document 2 and FIG. 2(a) on page 174 of the document 4. FIG. 8A is a schematic structural diagram, and FIG. 8B is a diagram showing problem. FIG. 9 is a schematic plan diagram showing an electrode of FIG. 8A and FIG. 8B.
The plasma monitoring system shown in FIG. 8A is provided with a plasma processing device 10. The plasma device 10 is a device which generates plasma 12 in a plasma chamber 11, which is in a vacuum state, by application of a high frequency (RF) bias voltage, and implements etching, film formation or the like on a monitoring subject 20 which is placed on a stage 13. Wiring 14 is connected to the monitoring subject 20. This wiring 14 leads out to outside the plasma chamber 11. A voltage supply 15, for applying a negative bias voltage (for example, −30 V) to the monitoring subject 20, and an ammeter 16, for measuring an induction current flowing in the monitoring subject 20, are connected in series with the wiring 14 that has been led out to the outside.
The monitoring subject 20 has a structure in which an electrode (for example, a polysilicon electrode 22 formed as a film with a substantially rectangular shape in plan view) insulated by a silicon dioxide film (SiO2 film) is formed on a wafer (for example, a silicon (Si) substrate 21), and a film to be used for an actual semiconductor device (for example, a silicon dioxide film 23) is formed on the polysilicon electrode 22. A portion of the silicon dioxide film 23 is opened up and a portion of the polysilicon electrode 22 is exposed, and the wiring 14 is connected to this exposed location via a wiring connection portion 24.
At a time at which monitoring of ultraviolet light UV is to be performed, when plasma processing of the monitoring subject 20 is being implemented by application of the RF bias voltage, pairs of holes h and electrons e are generated in the silicon dioxide film 23 on the polysilicon electrode 22 by the ultraviolet light UV emitted from the plasma 12. The negative bias voltage is applied to the polysilicon electrode 22 by the voltage supply 15, and thus the holes h are measured in real time by the ammeter 16, as an induction current. This induction current is monitored as a quantitative indicator of damage to the silicon dioxide film 23 that is caused by the ultraviolet light UV.
Further, as illustrated in FIG. 2(b) on page 174 of the document 4, a structure may be employed in which a surface of the silicon dioxide film 23, including the opening portion, is covered with an aluminum (Al) based metal film, in order to eliminate effects of ions i, electrons e or the like as much as possible.
As shown in FIG. 8B, when the monitoring subject 20 is exposed to the plasma 12, the plasma 12 has been separated into positive ions, and the electrons e and holes h act to electrostatically charge up the silicon dioxide film 23. Here, because the electrons e are much lighter than the holes h, the electrons e have higher speed, and large amounts of the electrons e are accumulated on the silicon dioxide film 23. As a result, a negative potential is generated at the top of the silicon dioxide film 23 by this electron charging. Then, the slower holes h having opposite charge to the electrons e reach the top of the silicon dioxide film 23, but amounts approximately canceling out the previously charging electrons e do not cause charging. Therefore, although the negative electrons e and positive holes h from the plasma 12 both eventually reach the top of the silicon dioxide film 23 and cause charging, an initial charging amount by the negative electrons e is large. Hence, a potential at the top of the silicon dioxide film 23 is in a stable state, and is a negative potential. This negative potential is referred to as a self-aligning bias Vdc.
Thus, it is known that usually, in a plasma processing process, a self-aligning bias Vdc is generally formed with a negative potential at a pattern surface. This potential varies greatly in value depending on conditions of the plasma processing. If the self-aligning bias Vdc is, for example, −80 V, when the feed-in bias is constant (−30 V in this case), a portion of the holes h that are generated in the evaluation subject silicon dioxide film 23 by the ultraviolet light UV will be attracted in a direction toward the surface of the silicon dioxide film 23 rather than the polysilicon electrode 22, and accurate measurement of induction currents is not possible, which has been a problem.